We recall that the irreducible representations of \({\rm Sp}(4,{\mathbb {Q}})\) 
are parametrized by the pairs \((a,b)\)  of integers with \(a\ge b \ge 0\) ,
and \(R_{a,b}\)  occurs in \({\rm Sym}^{a-b}(V) \otimes {\rm Sym}^b(\wedge ^2V)\) 
with \(V=R_{1,0}\) 
the standard representation of \({\rm Sp}(4,{\mathbb {Q}})\) . We refer to
[1]} for the representation theory.
